The present invention relates to a shield terminal for a coaxial cable.
A coaxial cable such as an antenna cable that is used for transmission of high-frequency signals is generally constructed such that an outer circumference of a core wire is covered with a braid via an insulator and furthermore, an outer circumference of the braid is covered with an insulating sheath. Then, as a connector for connecting such a coaxial cable to a mating device or coaxial cable, there has been known a shield terminal adapted to shield the coaxial cable from electrical noise such as electromagnetic wave and static electricity by earth connecting the braid of the coaxial cable to a mating connector (for example, refer to Patent Document 1).
As shown in FIG. 6, a coaxial cable shield terminal 100, which is disclosed in Patent Document 1, is made up of an outer conductor terminal 113 formed of a conductive sheet material and a shield body 114, and is secured to a terminal end portion of a coaxial cable W in which a sheath Wd, a braid Wc and an insulator Wb are cut for predetermined lengths, respectively, so that a core wire Wa, the insulator Wb and the braid Wc are sequentially exposed.
The outer conductor terminal 113 has a shield portion 113a which is formed into a cylindrical shape and a sleeve 113b which is provided in such a manner as to extend rearward of the shield portion 113a and is formed into a cylindrical shape having a smaller diameter than the shield portion 113a. The shield portion 113a receives in the interior thereof the core wire Wa exposed at the terminal portion of the coaxial cable W and an inner conductor terminal 111 mounted on the core wire Wa for electromagnetic shielding.
When the sleeve 113b is connected to the coaxial cable W, the insulator Wb exposed at the terminal portion of the coaxial cable W is fittingly inserted into the interior of the sleeve 113b, whereby the sleeve 113b is inserted between the braid Wc exposed at the terminal portion of the coaxial cable W and the insulator Wb situated radially inward of the braid Wc.
The shield body 114 is joined to the outer conductor terminal 113 in such a manner as to close an opening 113c at a rear end of the shield portion 113a, and a sheath clamping portion 114b provided at a rear end of the shield body 114 clamps a sheath Wd of the coaxial cable W by being compressed, whereby the shield body 114 is fixed to the terminal portion of the coaxial cable W together with the outer conductor terminal 113.
A braid clamping portion 114a, which is provided at a position on the shield body 114 which corresponds to the sleeve 113b when the shield body 114 is jointed to the outer conductor terminal 113, is compressed onto the sleeve 113b, so that the braid Wc, which covers the sleeve 113b, is held by the sleeve 113b and the braid clamping portion 114a, whereby the outer conductor terminal 113 and the shield body 114 are electrically connected to the braid Wc. Then, the braid Wc of the coaxial cable W is earth connected to a mating connector when the outer conductor terminal 113 is electrically connected to an earth terminal of the mating connector. Patent Document 1:JP-A-2003-297493 (pages 4 to 7, FIG. 3)
In the related coaxial cable shield terminal 100, however, the sleeve 113b is formed into the cylindrical shape having an inside diameter that is equal to or slightly larger than the outside diameter of the insulator Wb so that the insulator Wb of the coaxial cable W can fittingly be inserted into the sleeve 113b, and the construction of the sleeve 113b makes it difficult for the sleeve 113b to be deformed plastically.
Consequently, when the braid clamping portion 114a of the shield body 114 is compressed, the braid Wc which covers the sleeve 113b is only held by the sleeve 113b and the braid clamping portion 114a, and the insulator Wb, which is fittingly inserted into the interior of the sleeve 113b, is fixed in no case. Namely, the shield terminal 100 is fixed to the coaxial cable only by the sheath clamping portion 114b of the shield body 114, and there exists a possibility that the position of the shield terminal 100 is deviated with respect to the coaxial cable W. In case there occurs the positional deviation of the shield terminal, an unreasonable force is applied to the braid Wc which is held by the sleeve 113b and the braid clamping portion 114a, leading to a risk that the braid Wc is broken due to the force so applied, whereby a good electrical connection cannot be obtained between the shield terminal 100 and the braid Wc.
Furthermore, in the event that the clamping by the braid clamping portion becomes insufficient due to loosening that occurs in association with the springback of the braid clamping portion 114a after compression or with a lapse of time, the braid Wc cannot be held sufficiently between the sleeve 113b and the braid clamping portion 114a, leading to the risk that a good electrical connection cannot be obtained between the shield terminal 100 and the braid Wc.
In addition, in a coaxial cable which transmits high-frequency signals, in a case where the characteristic impedance of the coaxial cable is not aligned with the characteristic impedance of the shield terminal, a reflection of signal occurs. This reflection causes noise and energy loss. Then, the shield terminal 100, which constitutes a return circuit for signals, is made up by joining together the outer conductor terminal 113 and the shield body 114, which are separate components from each other, and there may be a case where an impedance disturbance occurs at the joint portion, and consequently, there has been a risk that the high-frequency wave characteristics of the coaxial cable W is deteriorated.